Add OpenMercantil API

[PabloCirre]

I would like to suggest OpenMercantil for the API collection.

OpenMercantil is an independent Spanish public-data API for company search, BORME registry-event timelines, downloads and source methodology.

Suggested YAML entry:

name: OpenMercantil
slug: openmercantil
description: Independent Spanish public-data API for company search, BORME registry-event timelines, downloads and source methodology.
categories:
  - Government
  - Business
  - Statistics & Data
type: REST
is_free: true
is_active: true
auth_type: none
rate_limits: Public no-auth endpoints have soft rate limits; expanded use may require account-level limits.
tags:
  - spain
  - companies
  - company-registry
  - public-data
  - open-data
  - borme
specification:
  type: OpenAPI
  url: https://openmercantil.es/openapi.json
links:
  - name: Docs / Website
    url: https://openmercantil.es/api/documentacion
  - name: Sources
    url: https://openmercantil.es/fuentes
  - name: OpenAPI
    url: https://openmercantil.es/openapi.json

OpenMercantil's mission is to contribute to open/public mercantile data access without profit-seeking intent. It is independent and informational, not the BOE, BORME or Registro Mercantil, and it does not replace official certificates or registry extracts.